1. Field of the Invention
This application relates to a fixing device that fixes a toner image onto a recording medium with heat and pressure. The application also relates to an image forming apparatus that includes the fixing device and prints an image in accordance with image data from an external device.
2. Description of the Related Art
An image forming apparatus, such as a printer, a copier, a facsimile machine or an electrophotographic color recorder, transfers a toner image corresponding to image data to a recording medium and fixes the toner image onto the recording medium with heat and pressure by a fixing device.
The fixing device includes a heat roller, an upper pressure roller, a lower pressure roller, a fixing belt, a temperature detector and a heater. The fixing belt is entrained about the heat roller and the upper pressure roller, and is heated by the heat roller. The temperature detector detects a temperature of the fixing belt at the portion that comes into contact with the heat roller. The image forming apparatus controls the heater based on the temperature detected by the temperature detector so as to keep the temperature of the heat roller constant.
In the fixing device, while the lower pressure roller is in pressure contact with the upper roller through the fixing belt, the recording medium with the toner image is advanced between the fixing belt and the lower pressure roller. As a result, the toner image, which adheres to a surface of the recording medium with mild static electricity, is fused on the recording medium with heat from the fixing belt and pressure from the lower pressure roller. Japanese Patent Laid-Open No. 2005-134646 discloses one such fixing device.
In the aforementioned fixing device, however, the temperature detector is provided in abutting contact with a surface of the fixing belt with which the recording medium comes into contact. Therefore, the surface of the fixing belt is liable to form scratches in which toner will become concentrated. This will cause adverse effects on print quality.
Meanwhile, a fixing device in which the temperature detector is provided in abutting contact with an inner surface of the fixing belt is also well known. In the fixing device, however, the fixing belt is liable to wear nonuniformly by friction between the temperature detector and the fixing belt, thereby causing variations in temperature of the fixing belt in its width direction. This also will cause adverse effects on print quality.